The human papovavirus JCV which heretofore has been grown only in human fetal glial cells was successfully adapted to growth in readily available human embryonic kidney cells. During the process of adaptation, defective viruses were generated which became the predominant type of virions in the adapted virus pool. The adapted virus formed plaques in HEK cells. However, cloned virus stocks derived after plaque-purification still yielded 2 or more species of viral DNA. The viral DNA was then molecularly cloned and analyzed by restriction endonuclease cleavage and by electron microscopic studies of heteroduplex molecules. From such analysis it was found that 2 kinds of DNA molecules were present in te stock, each with approximately 30% deletions in the early and late regions of the genomes. It is therefore postulated that the HEK adapted virus replicates efficiently in these cells by complementation of defective virions.